The present invention is related to Japanese patent application No. 2000-189598, filed Jun. 23, 2000; 2001-20163, filed Jan. 29, 2001, the contents of which are incorporated herein by reference.
The present invention relates to an electric drum brake device used for a vehicle brake and, more particularly, an electric drum brake device applied to a duo-servo braking force control type drum brake device.
The present applicant, in Japanese Patent Application No. 2000-52305, proposed a braking force control type drum brake device which suppresses a change in the braking force due to variations in the coefficient of friction. In this application, a lever for pressing a first shoe is connected to a lever for receiving a second shoe by a strut to provide a lever ratio of N. In this configuration, an anchor load applied to the lever for receiving the second shoe is fed back to the lever for pressing the first shoe to generate a braking torque by a resultant force to reduce a change in the braking force by a coefficient of friction between the shoe and a drum.
Here, as the lever ratio is smaller, the stability of the braking force is improved. However, motor power necessary for producing the braking force is increased. For this reason, it is necessary to select a lever ratio satisfying both and to set the lever ratio larger than 1.
Now, a braking torque when a vehicle moves forward and a braking torque when the vehicle moves backward, in the configuration disclosed in the prior application, is reviewed.
First, when the vehicle moves forward, the lever of the first shoe is an input and the lever of the second shoe is an anchor load. A braking torque is produced when the resultant force balances the motor output. For this reason, the braking torque TF when the vehicle moves forward is expressed by the following equation
TF=BEFxc3x97Fmxc3x97(1/(1+Fan/N))xc3x97Rxe2x80x83xe2x80x83(Equation 13)
where BEF is a brake effectiveness factor (mainly determined by a coefficient of friction between a shoe and a drum), Fm is the motor output, Fan is a force applied to the anchor member (mainly determined by BEF), and R is a radius of the drum.
On the other hand, when the vehicle moves backward, the lever of the second shoe is an input and the lever of the first shoe is the anchor load. The braking force is produced when the resultant force thereof balances the motor output. For this reason, a braking torque TR when the vehicle moves backward is expressed by the following equation:
TR=BEFxc3x97Fmxc3x97(1/(1+Fanxc3x97N))xc3x97R
However, when the lever ratio is set larger than 1, as described above, (1/(1+Fan/N)) greater than (1/(1+Fanxc3x97N)), that is, TF greater than TR and therefore the motor output is equal when the vehicle moves forward backward, the braking force when the vehicle moves backward is smaller than the braking force when the vehicle moves forward.
For this reason, in order to generate the same braking force when the vehicle moves forward and backward, the motor output must be set in accordance with the braking torque when the vehicle moves backward, thereby requiring the size of the motor to be increased.
In view of these and other drawbacks, a first object of the present invention is to provide a drum brake device capable of easily adjusting the braking force when a vehicle moves forward and when the vehicle moves backward.
Further, it is another object of the present invention to provide a drum brake device having a structure capable of generating the same braking force by the same motor output when a vehicle moves forward and backward.
Accordingly, a first aspect provides a driving force generating unit for generating a driving force responsive to the application of brakes on a vehicle; a first shoe having a first contact surface capable of contacting the inner peripheral surface of a rotary drum and being moved so that the first contact surface contacts the inner peripheral surface of the drum based on the driving force generated by the driving force generating unit; a second shoe having a second contact surface capable of contacting the inner peripheral surface of the drum and being moved in connection with the first shoe; a first lever having a contact portion with the first shoe and applying a load in the direction of the inner peripheral surface of the drum to the first shoe based on the driving force of the driving force generating unit; a second lever having a contact portion with the second shoe and applying a load in the direction of the inner peripheral surface of the drum to the second shoe based on the driving force of the driving force generating unit; a strut connected to the first lever and the second lever to connect the first lever to the second lever; an anchor member for receiving an anchor load generated by the first shoe or the second shoe; and a third lever connected to a portion of the first lever between the connection portion of the first lever with the strut and the contact portion of the first lever with the first shoe to feed back the load applied to the anchor member 18 to the first lever.
As described above, since the anchor load generated by the first shoe or the second shoe is fed back to the first lever via the third lever, either one of the anchor loads are fed back. In this way, the load applied to the first shoe or the second shoe by the first lever is controlled, and even if variations in a coefficient of friction occur, it is possible to prevent variations in a braking force and brake effectiveness factor.
In another aspect, if the brakes are applied when the drum is rotated in the first direction F, the first and second contact surfaces are put into contact with the inner peripheral surface of the drum. And, the second shoe is pressed by the first shoe to generate an anchor load which is applied to two portions of the contact portion of the second lever with the second shoe and the anchor member, and if the brakes are applied when the drum is rotated in the second direction B, the first and second contact surfaces contact with the inner peripheral surface of the drum and the first shoe is pressed by the second shoe to generate an anchor load which is applied to two portions of the contact portion of the first lever with the first shoe and the anchor member.
In another aspect, if the brakes are applied when the drum is rotated in the first direction, the third lever receives the load applied to the anchor member to apply a load to the first lever in the direction in which the load applied to the first shoe by the first lever is decreased, and if the brakes are applied when the drum is rotated in the second direction, the third lever receives the load applied to the anchor member to apply a load to the first lever in the direction in which the load applied to the first shoe by the first lever is increased.
In another aspect, a fourth lever is provided which can be swung around a predetermined portion and has the anchor member arranged at one side of the predetermined portion and is connected to the third lever at the other side of the predetermined portion. It is possible to feed back the anchor load from the third lever to the first lever by the fourth lever having such a configuration.
In another aspect, by connecting the end portion of the first lever, opposite to the contact portion thereof, with the first shoe across the connection portion thereof to the strut to an output lever 7 to which the driving force of the driving force generating unit is outputted, it is possible to drive the first lever by the driving force generating unit.
In another aspect, the equations for calculating brake effectiveness factors BEFembf, BEFembr in the first and second directions are formed based on the relationship of the forces generated at the various portions when the drum is rotated in the first and second directions and that the lever ratios of the first, second, and fourth levers are set based on the equations.
In this way, it is possible to set the lever ratios of the respective levers based on the equations for calculating the brake effectiveness factors BEFembf, BEFembr.
In this case, in another aspect, if the ratios of the respective levers are set so that the brake effectiveness factor BEFembf is equal to the brake effectiveness factor BEFembr, it is possible to obtain the same braking force when the vehicle moves forward and backward.
In another aspect, the transfer portions of force by a link mechanism including the first shoe, the second shoe, the first lever, the second lever, the third lever and the strut are arranged on the same plane perpendicular to an axle. According to this configuration, it is possible to transfer a load by the link mechanism on the same plane and hence to improve the transfer efficiency of the load (force) and to suppress the hysteresis of the braking torque to the torque generated by the driving force generating unit.
For example, in another aspect, the transfer portions of the respective forces in the contact portion of the first shoe with the first lever, the connecting portion 12a of the first lever to the strut, the contact portion 9 of the second shoe with the second lever, and the contact portion of the second lever with the strut are arranged on the same plane perpendicular to the axle.
In another aspect, between a plurality of driving balls contacting both a screw shaft portion and a nut portion, provided in a speed reducing mechanism, are disposed contact preventing members, 60c that prevent the respective driving balls from contacting. Accordingly, the sliding resistance between the driving balls is reduced and the transfer efficiency of force of the speed reducing mechanism is improved and the above-mentioned effect of suppressing the hysteresis is provided.
In another aspect, a spacer ball can be used that is smaller than the driving ball as the contact preventing member. Further, according to another aspect, a retainer shaped like a drum in cross section can be used as the contact preventing member.
In another aspect, the number of circulations (the number of circuits) of sets of driving balls which are mounted spirally in contact with both the screw shaft portion and the nut portion is plural. According to this configuration, it is possible to elongate the effective length of the shaft of the screw shaft portion with respect to the diameter of the shaft thereof. Hence, the ball screw can be prevented from tilting. For this reason, it is possible to improve the transfer efficiency of force of the speed reducing mechanism and to produce the above-mentioned effect of suppressing the hysteresis.
In another aspect, the speed reducing mechanism has a hollow housing in which a screw shaft support portion for driving the screw shaft portion and the nut portion can slide, and which has a hole through which the screw shaft support portion is arranged, a bearing 40 which is disposed in the hole of the housing 38 and supports the screw shaft support portion, lubricating oil or grease packed in the space formed by the housing and the nut portion, and a sealing member which is arranged with the bearing in the hole of the housing.
Since the sealing member is provided in this manner in the hole of the housing where the bearing is arranged, it is possible to prevent the lubricating oil from leaking outside the housing. For this reason, it is possible effectively rotate the screw shaft portion and to improve the transfer efficiency of force of the speed reducing mechanism and to produce the above-mentioned effect of suppressing the hysteresis.
In this respect, the reference characters of the respective units and members designate the relationship corresponding to the specific units or members in the preferred embodiments described below.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In the drawings: